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The adaptive immune response is initiated in secondary lymphoid organs by contact between antigen-bearing dendritic cells (DCs) and antigen-specific CD4⁺ T cells. However, there is scant information regarding the single cell dynamics of this process in vivo. Using two-photon microscopy, we imaged the real-time behavior of naive CD4⁺ T cells and in vivo–labeled DCs in lymph nodes during a robust T cell response. In the first 2 h after entry into lymph nodes, T cells made short-lived contacts with antigen-bearing DCs, each contact lasting an average of 11–12 min and occurring mainly on dendrites. Altered patterns of T cell motility during this early stage of antigen recognition promoted serial engagement with several adjacent DCs. Subsequently, T cell behavior progressed through additional distinct stages, including long-lived clusters, dynamic swarms, and finally autonomous migration punctuated by cell …